This invention relates to multiplexing a plurality of various rate subchannels onto a fixed rate channel and more particularly to defining an optimal frame structure that accomplishes this.
U.S. Pat. No. 4,617,658 issued Oct. 14, 1986 to Stephen M. Walters, co-inventor herein, and which is incorporated herein by reference, describes a framing structrure that enables a plurality of various rate subchannels to be multiplexed onto a fixed rate channel. As described therein, the frame structure consists of j sets of i-tuples for a total of ij bits per frame, wherein the parameters i and j are mathematically determined as a function of the rates of the various subchannels and the rate of the fixed channel. For j-1 of the i-tuples, i-1 bits are used for information and one bit at either end is set ONE (or ZERO). In one of the i-tuples, all i bits are set ZERO (or ONE). In the (i-1)(j-1) information bit positions per frame, an integer number of information bits are distributed so that the required rate for each subchannel is provided. Maintaining proper framing of a data stream consisting of successive of these frames is readily achieved by monitoring the data stream for i successive ZEROes (or ONEs) followed or proceeded by a ONE (or ZERO), which can only occur in the data stream at the i-tuple which is set all ZERO (or ONE) per frame regardless of the data pattern of the bits in the information slots. Advantageously, framing can be detected with simple and inexpensive hardware and reframing following an out-of-frame condition can be achieved in only one frame.
By employing the techniques described in the aforenoted patent, parameters i and j can always be found for any combination of subchannel rates and fixed channel rate as long as the sum of all the subchannel rates is strictly less than the rate of the fixed channel and the fixed channel rate and the subchannel rates can be expressed as rational numbers. The resultant frame, however, consisting of ij bits, may not in all cases be the frame having the fewest number of bits achievable which uses the framing marker of i ZEROes (or ONEs) proceeded or followed by a ONE (or ZERO), and also has sufficient information slots for an integer number of bits from each subchannel to maintain the subchannel rates. Since reframe time following an out-of-frame condition is directly dependent upon frame length, minimization of frame length is desirable.